In the past, the nuclear magnetic resonance (NMR) and electron spin resonance (ESR) phenomenon have been utilized by structural chemists and physicists to study, in vitro, the molecular structure of atoms and molecules. Typically, NMR and ESR spectrometers utilized for this purpose were designed to accommodate relatively small samples of the substance to be studied.
More recently, however, NMR has been developed into an imaging modality utilized to obtain images of anatomical features of live human subjects, for example. Such images depicting parameters associated with nuclear spins (typically hydrogen protons associated with water in tissue) may be of medical diagnostic value in determining the state of health of tissue in the region examined. NMR techniques have also been extended to in vivo spectroscopy of such elements as phosphorus and carbon, for example, providing researchers with the tools, for the first time, to study chemical processes in a living organism. The use of NMR to produce images and spectroscopic studies of the human body has necessitated the use of specifically designed system components, such as the magnet, gradient and RF coils.
By way of background, the nuclear magnetic resonance phenomenon occurs in atomic nuclei having an odd number of protons and/or neutrons (i.e., a quantum mechanical asymmetry in the nucleus). Due to the spin of the protons and neutrons, each such nucleus exhibits a magnetic moment, such that, when a sample composed of such nuclei is placed in a static, homogeneous magnetic field, B0, a greater number of nuclear-magnetic moments align with the field to produce a net macroscopic magnetization M in the direction of the field. Under the influence of the magnetic field B0, the magnetic moments precess about the axis of the field at a frequency which is dependent on the strength of the applied magnetic field and on the characteristics of the nuclei. The angular precession frequency, w, also referred to as the Larmor Frequency, is given by the equation w=g B, in which g is the gyromagnetic ratio (which is constant for each NMR isotope) and wherein B is the magnetic field (B0 plus other fields) acting upon the nuclear spins. It will be thus apparent that the resonant frequency is dependent on the strength of the magnetic field in which the sample is positioned.
The orientation of magnetization M, normally directed along the magnetic field B0, may be perturbed by the application of magnetic fields oscillating at or near the Larmor frequency. Typically, such magnetic fields designated B1 are applied orthogonal to the direction of magnetization M by means of radio-frequency pulses through a coil connected to radio-frequency-transmitting apparatus. Magnetization M rotates about the direction of the B1 field. In NMR, it is typically desired to apply RF pulses of sufficient magnitude and duration to rotate magnetization M into a plane perpendicular to the direction of the B0 field. This plane is commonly referred to as the transverse plane. Upon cessation of the RF excitation, the nuclear moments rotated into the transverse plane begin to realign with the B0 field by a variety of physical processes. During this realignment process, the nuclear moments emit radio-frequency signals, termed the NMR signals, which are characteristic of the magnetic field and of the particular chemical environment in which the nuclei are situated. The same or a second RF coil may be used to receive the signals emitted from the nuclei. In NMR imaging applications, the NMR signals are observed in the presence of magnetic-field gradients which are utilized to encode spatial information into the NMR signal. This information is later used to reconstruct images of the object studied in a manner well known to those skilled in the art.
High-field imaging (≧3 T), while being advantageous for increased signal-to-noise ratios (SNR), is a challenging task that requires special instrumentation. Constructing efficient volume coils for high-field body imaging is difficult due to low SNR and significant power deposition factors associated with the large fields-of-view (FOV) of these devices. As a result, only limited feasibility assessment efforts have been reported. On the other hand, surface coils, which have restricted FOVs and, higher efficiencies and lower power deposition factors, do not provide homogeneous RF magnetic fields (B1 fields) needed in many experiments. Alternative coil designs for high-field MRI imaging applications, therefore, become highly desirable.
For purposes of this application the term open as it applies to volume coils is intended to refer to those coils which are sufficiently open to avoid magnetic or electrical coupling between the end resonant elements. The idea of constructing open volume MRI coils for imaging of large objects, such as a human body, has been explored previously for experiments at 1.5 T. Additionally, some attempts to construct open head coils of very different geometry have also been made for experiments at higher fields, for example U.S. Pat. No. 5,744,957 to Vaughan, hereby incorporated by reference. However, these attempts are based on a fundamentally different design. A number of imaging applications can be greatly benefited by the use of open volume coils. These devices provide sufficiently large homogeneous B1 field regions, while maintaining higher efficiency and sensitivity as well as relatively low power deposition factors, because their FOVs are substantially reduced. They also improve patient comfort and accessibility, a feature that is especially important in functional magnetic resonance imaging (fMRI), where additional stimulation equipment is used inside volume coils.
It is preferable to operate volume coils in quadrature because of the transmission efficiency and the SNR increase by a factor of 21/2. At high field strength, an additional benefit of quadrature operation is the reduction of RF penetration artifacts that distort images of conducting objects (such as a human body or head) which are most pronounced when linear devices are used. Quadrature coil operation is easily achieved in a full-volume coil by simultaneously driving a naturally occurring degenerate pair of modes. However, the situation is different for an open half-volume coil (TEM or birdcage), in which no frequency-degenerate modes naturally exist. To achieve quadrature operation, therefore, two orthogonal modes must be selected, explicitly made degenerate and independently driven.
TEM volume coils have become a well-established alternative to conventional birdcage designs for imaging at high fields due to a number of advantages. Because the inductances of the elements comprising birdcage coils increase with the coils' overall sizes, unreasonably low value capacitors are required to resonate the large body birdcage coils. Size scaling of TEM coils, on the other hand, is much easier, since the area of the elements comprising a TEM coil can be controlled by adjusting the distance between the coil's legs and its shield. Additional advantages of the TEM coils include absence of end ring currents and better sensitivity. While open half-volume linear and quadrature birdcage coils have been constructed in the past for low field (1.5 T) applications, successful development of similar TEM devices have not been reported. A partial solution was provided by removing a single top element in a 16-element TEM resonator. For the reasons discussed below this coil cannot be considered an open coil but is rather a closed full-volume coil in which the current distribution patterns became somewhat distorted by the removal of an element and could be compensated by appropriate adjustments to restore the full-volume coil quadrature operation.
Open half-volume coils, as compared with the closed full-volume devices, can provide significantly higher transmission efficiency because their FOVs are decreased approximately in half. The value of the RF magnetic field B1 per unit of applied power can be obtained, according to:             B      1              P        ∼                    Q        ·        η                    ω        ·                  V          s                    where P is the input power, Q is the coil's quality factor, ω is the resonance frequency, η is the magnetic field filling factor and Vs is the sample volume seen by the coil, directly related to the FOV of the coil. Thus, the decrease in the FOV leads to the increase in the transmit efficiency. Additionally, since it is easier to fit objects inside open coils, their magnetic field filling factors, η, are higher, leading to a further increase in the B1 value according to the equation. Substantial SNR improvements are also achieved with these coils, in which a simultaneous increase in the detected signal strength and a reduction in the noise occur. The signal enhancement can be understood if one considers the reciprocity principle that states that the amount of signal picked up by the coil is proportional to the amount of the B1 field produced per unit of the input power in the transmit mode. Furthermore, a noise reduction results from the smaller FOV of the open coils relative to conventional closed volume coils, which, when operated in receive mode, collect noise from the areas outside of the region of interest for the study.
A TEM coil consists of a number of identical coupled resonant elements producing several modes resonating at different frequencies. The number of modes in a chain of coupled elements is equal to the number of elements. In a closed TEM coil, some of the modes become degenerate and the number of distinct resonant frequencies is reduced. The second lowest resonance frequency of this coil corresponds to a frequency-degenerate pair of modes with currents flowing in the elements such that their amplitudes are modulated sinusoidally going from one rung to the next with one period over a 360° revolution by the azimuthal angle that defines the element. Such distribution of currents has been shown to create homogeneous B1 fields inside the coil. Quadrature coil operation is easily achieved in a closed TEM coil by simultaneously driving these two orthogonal degenerate modes by connecting in series to two 90°-separated resonant elements, producing a circularly polarized homogeneous B1 field.
Analogous to a typical closed TEM, the open TEM 10 shown in FIG. 1A, and having elements 1–7 can be viewed as a half-wave resonator where a standing wave is formed in the radial direction. There are, however, some very important differences between the mode current distributions in the elements forming these two types of coils. In a closed TEM coil, the current distributions are modulated sinusoidally going around the coil, such that an integer number of full periods fit for one complete revolution. In an open TEM coil, no coupling exists between the first and the last elements in the chain, so this boundary condition changes, requiring that an integer number of half-periods fit between the first and the last element. Modes are formed at different frequencies according to the number of the half-periods. Thus, the lowest frequency corresponds to zero half periods (currents in all elements are in phase), the second lowest corresponds to one half-period (here referred to as the surface mode of the coil), third lowest—to two half-periods or one period (here referred to as the butterfly mode of the coil), etc. FIG. 1B shows theoretical current distributions and B1 patterns for the surface and the butterfly modes for each of elements 1–7. In a half-volume TEM coil no frequency-degenerate modes naturally exist. Therefore, to achieve quadrature operation, a completely different approach has to be taken, in which two orthogonal modes corresponding to the most homogeneous orthogonal B1 fields appearing at neighboring frequencies (second and third lowest) are selected, explicitly made degenerate and independently driven.
As was mentioned above, attempts have been made in the past to create an open volume coil configuration by simply removing an element from a closed volume coil and relying on the adjustments made in the remaining elements to restore full-volume coil quadrature operation. This is possible because the orientation of the elements in a TEM coil is such that the through-space inductive coupling is still in effect when one element of a closed densely spaced TEM coil is removed. Therefore, this coil's operation was in principle the same as in a regular closed TEM coil, where the current distribution patterns became somewhat distorted by the removal of an element. That distortion could be compensated by appropriate adjustments of other elements to restore the closed full-volume coil quadrature operation, and quadrature driving of two 90°-separated resonant elements still could be applied. However, removing more than one element would substantially decrease inductive interaction between the two ending elements and change the boundary conditions from a closed coil state (requiring an integer number of full periods of the sinusoidal current distribution around the coil) to an open state (requiring a integer number of the half-periods). Completely different modes would emerge as a result. Therefore, the “closed coil” state cannot be restored by simple adjustments in the elements in this case, making it impossible to utilize quadrature driving as in closed full-volume coils.
Taken against this background and the history of development in this field, a need exists for an appropriately driven quadrature half-volume TEM coil.